feat(demo): add multi-tab demo with dual BimEngine instances

New demo page with two independent BimEngine instances side by side,
draggable divider for resizing, and proper SDK-layer resize handling.
Replaces the iframe-based approach now that ManagerRegistry is instance-based.
Removes unused panelViewer vite config entry.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

demo/vite.config.js

@@ -16,7 +16,9 @@ export default defineConfig({
     outDir: 'dist',
     rollupOptions: {
       input: {
-        main: resolve(__dirname, 'index.html')
+        main: resolve(__dirname, 'index.html'),
+        viewer: resolve(__dirname, 'viewer.html'),
+        multiTab: resolve(__dirname, 'multi-tab.html')
       }
     }
   }